docs: def validate(self, form, extra_validators=tuple()):
          '''Perform validation only if data has been submitted'''
          if not self.has_data:
              return True
          if self.is_list_data:
              if not isinstance(self._formdata[self.name], (list, tuple)):
                  return False
          return super(NestedModelList, self).validate(form, extra_validators)